The present invention relates to the fractionation of mixed populations of triglycerides, more particularly triglycerides contained in anhydrous milk fat.
Mixtures of mixed triglycerides, and more particularly anhydrous milk fat (AMF), have proven utility as a food component when separated into fractions containing different populations of triglycerides. However, difficulty in processing milk fat and fractionating the triglycerides in commercial scale has restricted its use in the food industry in the United States. In Europe, where fat fractionation is carried out in a batch process, the practice is more common.
The properties of milk fat vary greatly depending upon the season, the region, the breed of dairy cow and even the animal feed. Since AMF is composed of an exceedingly large variety of fat molecules in the form of triglycerides, the properties of AMF are a normalized average of the characteristics of its individual components. A homogenous fraction of any one or a group of related triglycerides has different physical properties (e.g. different melting points, blending properties with baking or confectionery ingredients, and textural characteristics). The characteristics of milk fat and associated physical properties such as melting point have been studied, but cannot be precisely determined from one batch to the next. In general, ordinary AMF has a wide melting range which makes it unsuitable for many food applications.
Fractionation of milk fat is a process of separating the triglyceride components of milk fat on the basis of their melting points. As a result of the varying characteristics of the milk fat, the fractionation of milk fat is as much art as it is science because the many variables are difficult to control in conventional batch procedures.
Fractionation is accomplished by the selective crystallization of the triglyceride components of a mass of melted AMF. The energy for crystallization is equal to the difference between the melting point of the triglycerides and the actual solution temperature. Crystallization from the melt is characterized by the formation of a lattice structure as the triglycerides molecules undergo a phase transition from liquid to solid. Crystallization occurs in two stages. The first is nucleation where embryonic crystals referred to as nuclei are formed. The second stage is crystal growth which involves diffusion of the triglycerides into the growing crystal lattice structure.
Interestingly, it is found that the actual distribution of fatty acids contained in the various fractions obtained at high, medium, or low temperatures from a melt does not differ as much as might be expected. This appears to be explained by the degree of saturation and chain length of the fatty acids on the intact triglyceride. Fractions having a high temperature (40-50xc2x0 C.) melting profile generally have a total carbon chain length for the three glycerols (acyl carbon number) of C44 to C54, and those having a low temperature ( less than 20xc2x0 C.) melting profile generally have a total carbon (C) chain length from C26 to C42. The relation of the triglyceride composition of different fractions to the crystalline forms and polymorphisms and to various product applications is reviewed in E. Deffense, JAOCS, 70: 1193 (1993) which is hereby incorporated by reference.
Currently, most milk fat fractionation is performed using the Tirtiaux dry fractionation process. This process essentially involves crystallization by cooling of a commercial batch of milk fat. Depending on capacity of the equipment from 10 to 100 tons/day of AMF may be processed. The crystallization is performed by heating the AMF to create a stabilized melt and then cooling and agitating the melt and collecting a solid fraction crystallizing at a relatively high temperature. The process is repeated to collect fractions crystallizing at successively lower temperatures.
In the prior art, it is believed that the agitation of milk fat during cooling improves the crystallization, thereby improving the fractionation. The batch of milk fat is slowly cooled and gently agitated in a slow, controlled process, taking up to 24 hours for a single batch. Nucleation and crystallization both occur in an agitated environment.
In the Tirtiaux method, the hard fractions obtained are collected over a range of upper temperatures to form hard stearin, and a liquid portion known as olein. In a repeat of the crystallization step on the olein fraction, a second soft stearin and olein fraction are obtained. This olein can then be further cooled to lower temperatures to create fractions with intermediate (20-30xc2x0 C.) and low ( less than 15xc2x0 C.) melting points. These fractions can be used in various compositions to yield milk fat/oil homogenates with improved properties in confectioneries and shortenings.
The present invention overcomes the limitations of the prior art by more precisely controlling the triglyceride fractionation process. Additionally, the present invention makes triglyceride fractionation more commercially usable because the process may operate alternatively in continuous flow mode or batch mode, in contrast to prior art processes which are limited to batch mode only. As a result, a significantly shorter residence time is required (e.g. 4 or 5 hours in the present invention instead of up to 24 hours in the Tirtiaux process). Also, some of the equipment components may be made smaller and less costly. Further, and perhaps most importantly, the triglyceride fractions may be more precisely controlled to achieve different and reproducible properties in the final product.
The present invention separately and precisely controls induced nucleation (which is a nuclei generation procedure by imposing an additional energy to the metastable melt) and crystal growth of the triglycerides. By separately controlling triglyceride nucleation and crystal growth, it becomes possible to utilize separate specialized components, and configure them in series. The present invention thus provides a continuous process for triglyceride fractionation.
The present invention may also be used in conjunction with a seeding process, which is another means of induced nucleation. In the seeding process, a particulate is added to the melt to provide a site for crystal formation. Such seeding may result in increased output and efficiency of the process, and decreased energy input. Proper use of seeding is made possible by the precise control of the separate aspects of the crystal formation and growth processes.
It may be more efficient to fractionate the triglyceride mass in separate iterations of the process. In this multistage process, individual passes through the apparatus may be tailored to fractionating out more specific portions of the triglyceride mass to obtain different solid and liquid fractions, and higher processing rates may be obtained. In a preferred embodiment, 2 or 3 iterations of the process may be used, each producing a solid fraction with a desired MMT (successively lower with each iteration) and a liquid fraction.
In accordance with the present process, a mass of mixed triglycerides is heated to form a stabilized homogenous melt at about 65-75 degrees C. The melt is pumped through a conventional heat exchanger of high capacity (capable of reducing the temperature from about 70 degrees Centigrade to nucleation temperature in tens of seconds) so as to supercool the mass of triglycerides below its melting point. For a continuous process, the melt is preferably cooled at about 20xc2x0 C. per minute. In a batch process, the melt is preferably cooled at a rate of about 1.0xc2x0 C. per minute. The supercooled melt is nucleated as a separate, isolated step in a relatively small volume under turbulent conditions while agitating sufficiently to form prenucleation clusters (regions throughout the melt of increased density/viscosity), followed by nuclei formation under appropriate shear conditions which attain or exceed the delta G (Gibbs free energy) threshold for critical size, the minimum size that crystalline particles can survive in the solution and not be dissolved. The nucleated mass of triglycerides is then transferred to a larger crystallizer where temperature is controlled (usually at temperatures slightly elevated above the nucleation temperature), and the crystallization process continues at a fast rate in a static environment and the crystals experience a structure transformation to produce higher quality crystals than are attained during further agitation and cooling. Alternatively, the above method may be practiced in a batch mode with the proper equipment.
The slow sedimentation step allows a more uniform crystallization to form a fraction of more uniform and predictable content, as indicated by correlation of the triglyceride profiles of the starting material with the composition of the final product obtained after collection of the crystalline sediment. A significant advantage of the present process is that the nucleation step, which in large batch is difficult to control because of the generation of highly variable shear forces throughout the batch, is now isolated in a small volume where the forces are more uniform throughout the solution.
In the apparatus of the invention, the functions of melting, supercooling, nucleation, and crystal growth are preferably carried out in separate vessels, in order to gain the requisite control over individual steps. In this way an appropriate level of control can be exerted over each particular step as to the individual requirement. Thus, the melt step utilizes a large batch tank that is steam jacketed, and equipped with an agitator. This configuration can keep all the milk fat components in a homogeneous mix at a constant temperature.
The apparatus also includes a heat exchanger of conventional construction, having a capacity to rapidly cool the melt to supercooled temperatures in about tens of seconds from about 70 degrees Centigrade to about nucleation temperature. The exchanger is sized to achieve reduction in temperature of a volume of melt equal to one batch of processing in a nucleator chamber. The nucleator chamber is designed according to a shape and volume, so that with suitable stirring means disposed therein, relatively uniform shear forces are distributed within the volume of melt contained in the chamber. The shape, volume, and shape and position of baffles and stirring means are determined in accordance with conventional engineering principles of fluid dynamics taking into account the viscosity and other flow parameters of the melt, and the velocities of flow needed to attain the range of Reynolds number of agitation calculated for melts of varying triglyceride content.
In another embodiment of the apparatus, the functions of melting, supercooling, nucleation and crystal growth are carried out in the same vessel, herein referred to as a combination melting/nucleating/crystallizing unit or simply combination unit. The combination unit includes a vessel with a steam jacket for melting, a heat exchanging means for cooling the melt and an energy input means for nucleation and agitating the melt. The heat exchanging means is preferably capable of decreasing the temperature of the melt at a rate of about 1.0 degree Centigrade per minute. Most preferably, the heat exchanging member is capable of decreasing the temperature of the mass of mixed triglycerides from about 70 degrees Centigrade to the nucleation temperature as rapidly as possible or in about 20 minutes. The energy input means is capable of providing the delta G (Gibbs free energy) threshold for critical size for nucleation. The shape and position of the energy input means with respect to the baffles created by the heat exchange means are determined in accordance with conventional engineering principles of fluid dynamics taking into account the viscosity and other flow parameters of the melt, and the velocities of flow needed to attain the range of Reynolds number of agitation calculated for melts of varying triglyceride content.